The NAVASTAR Global Positioning System (GPS) is a United States Defense Department satellite-based radio-navigation system transmitting information from which extremely accurate navigational information can be computed including the time, the user's three-dimensional position anywhere on or near the Earth, and the user's three-dimensional velocity. When fully operational, the GPS is planned to employ 18 satellites evenly dispersed in three, inclined, 12-hour circular orbits chosen to insure continuous 24-hour coverage worldwide. Each satellite carries extremely accurate cesium and rubidium vapor atomic clocks providing timing information. Additionally, each satellite is provided clock correction and orbital information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band carrier signals including an L1 signal having a frequency of 1575.42 MHz (also referred to as 1540 fO where fO is 1.023 MHz) and an L2 signal having a frequency of 1227.6 MHz (1200 fO). The L1 and L2 signals are biphase modulated by pseudo-random noise (PRN) codes. The PRN codes facilitate multiple access. Since each satellite uses different PRN codes, a signal transmitted by a particular satellite can be selected by generating and matching (correlating) the corresponding PRN code pattern. Additionally, the PRN codes facilitate signal transmit time measurements which can be made by measuring the phase shift required to match the code. Both of the carrier signals (L1 and L2) are modulated by a PRN code which is referred to as a precision (p) code. The p PRN code, which is intended for military purposes, is a relatively long, fine-grained, precision code having a clock rate of 10.23 MHz (10 fO). The L1 carrier signal is additionally modulated by a PRN code which is referred to as a clear/acquisition (C/A) code. The C/A PRN code, which is intended for rapid signal acquisition and for commercial purposes, is a relatively short, coarsegrained code having a clock rate of 1.023 MHz (fO) and a code length of 1023 bits (one millisecond). A full bit (chip) of C/A PRN code, phase delay corresponds to a distance of 293 meters. In addition to the PRN codes, both of the signals (L1 and L2) are, continuously, biphase modulated by a 50 bit per second, 1500 bit long, navigation data bit stream. The navigation data bit stream includes information as to the status and emphemeris of all satellites, parameters for computing the particular satellite clock, and corrections for atmospheric propagation delays.
Disclosed in the U.S. Pat. No. 4,754,465 of Charles R. Trimble is a Global Positioning System Course Acquistion Code Receiver suitable for computing the time, position, and velocity information. Unfortunately, to accurately compute the velocity information, the above mentioned receiver requires the use of a relatively stable, and, thus, relatively expensive, local oscillator (26).
Specifically, to compute the velocity information, the above mentioned receiver measures, in turn, the apparent doppler frequency shift of the 1540 fO (L1) carrier signal transmitted by each of four satellites. From the apparent doppler frequency shift information, from information as to the position of the receiver, and from information as to the position and the velocity of each of the four satellites, the receiver solves four equations in four unknowns to obtain the velocity information.
Each of the four equations are of the form: EQU D.sub.i =(v.sub.i .multidot.r.sub.i +v.sub.u .multidot.r.sub.i)(2f.sub.c /c)+(f-f.sub.o) (1)
where:
D.sub.i is the apparent doppler frequency shift in Hz of the 1540 fO carrier signal of the ith satellite, as measured at the receiver;
v.sub.i is the (three dimensional) velocity of the ith satellite in meters per second, as calculated from the navigational data transmitted by the satellite;
r.sub.i is a unit vector pointing from the ith satellite to the receiver, as calculated from the position of the receiver (previously calculated) and the position of the satellite (calculated from the navigational data transmitted by the satellite);
v.sub.u is the (three dimensional) velocity of the receiver in meters per second, the three components of which (v.sub.x, v.sub.y, and v.sub.z) represent three of the four unknowns;
f.sub.c is the satellite (L1) carrier frequency 1540 fO in Hz;
c is the speed of light in meters per second; and (f-f.sub.O) is a receiver frequency offset (error) in Hz, introduced, at least in part, by the receiver local oscillator (26), the fourth unknown.
In the above mentioned receiver, in computing the velocity information, it is assumed that the (f-f.sub.O) receiver frequency offset (error) is constant (stable) during the period of time required to make the four doppler frequency shift measurements. To the extent that the (f-f.sub.O) receiver frequency offset (error) changes from one doppler frequency shift measurement to the next, an error is introduced in the velocity information computed. Thus, to minimize the error, the above mentioned receiver uses a relatively stable, and, thus, relatively expensive, local oscillator (26).